Long-Lived Hole Stabilized at a Triphenylamine Core and Shielded by

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ARTICLES Long-Lived Hole Stabilized at a Triphenylamine Core and Shielded by Rigid Phenylazomethine Dendrons: A Pulse Radiolysis Study Akinori Saeki,*,† Shu Seki,‡ Norifusa Satoh,§ Kimihisa Yamamoto,*,§ and Seiichi Tagawa† The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, and Faculty of Science and Technology, Keio UniVersity, Yokohama 223-8522, Japan ReceiVed: June 16, 2008; ReVised Manuscript ReceiVed: September 24, 2008

The optical properties and reactivity of one-electron oxidized states (radical cations) of dendrimers are investigated in benzonitrile solutions by nanosecond pulse radiolysis. The hole stabilized at the triphenylamine (TPA) core is effectively shielded by a rigid dendritic phenylazomethine (DPA) shell of four generations, leading to an extension of its lifetime by nearly 2 orders of magnitude in comparison with a core radical cation without dendrons. A continuous red shift of the peak in the photoabsorption spectrum in the visible region and a decrease in the extinction coefficients (oscillator strengths) are found with increasing dendrimer generation number. These experimental observations are compared to the results of time-dependent density functional theory. It is suggested that bulky, rigid, insulating DPA dendrons shield against outer reactants and stabilize hole at the TPA core. Correlating the dendrimer generation number with the optical properties and reactivities of the radical cations could shed light on fundamental aspects of structurally defined nanoenvironments having hyperbranched entities. 1. Introduction Dendrimers are three-dimensional hyper-branched structures that exhibit a wide variety of functionalities for organic lightemitting diodes (OLEDs),1 light-harvesting antennae,2 charge transport materials,3 nanocatalysts,4 reaction flasks,5 and drug delivery systems.6 Energy migration from the exterior shell to the interior core plays an important role in the light-harvesting architecture, mimicking photosynthesis in chlorophyll. Enhancement of the charge transport efficiency is essential for applications to OLEDs and organic thin-film transistors. By inducing a positive charge at the dendrimer shell by photolysis, Hara and Majima et al. have demonstrated hole transfer from benzyl ether dendrons to the stilbene core, suggesting an effective shielding of the core radical cation against chloride ions.7 Ghaddar et al. have reported photoabsorption spectra of radical anions from a ruthenium core in a biphenyl dendron by pulse radiolysis and found a spectral shift and decrease in absorbance.8 Dendrimers having the dendritic phenylazomethine (DPA) shown in Figure 1 possess unique properties: stepwise radial complexation of metal ions at the DPA sites of which the location and number are precisely controlled by the gradient electron density of the imine group specific to the generation number.9 A triphenylamine core (TPAc) with DPA dendrons has demonstrated enhanced hole transport in electroluminescent devices and solar cells, resulting from wet-process and metal* To whom correspondence should be addressed. Phone: +81-6-68798502. Fax: +81-6-6876-3287. E-mail: (A.S.) [email protected]; (K.Y.) [email protected]. † The Institute of Scientific and Industrial Research, Osaka University. ‡ Graduate School of Engineering, Osaka University. § Keio University.

Figure 1. Chemical structures of the core and dendrimers.

assembling properties,3a,9c where the metal acts as a hole-transfer mediator. Furthermore, the generational growth of dendrons enhances these performances, probably led by stabilization of hole at the core with the dendron shells. However, the optical properties and stabilities of charged-state TPA-DPA dendrimers have not been well explored, since conventional chemical oxidation results in broad and structure-less spectra due to the effects of decomposition and/or counterions. Here we report a systematic investigation of the optical properties of a hole at the TPA core with DPA dendron of up to four generations. TPA-DPA dendrimers and control samples are prepared and transient photoabsorption spectra of their positively charged states are measured in benzonitrile solution by nanosecond pulse radiolysis. We demonstrate for the first time a correlation of the dendrimer generation with the extinction

10.1021/jp805266v CCC: $40.75  2008 American Chemical Society Published on Web 11/13/2008

Long-Lived Hole Stabilized at a Triphenylamine Core

J. Phys. Chem. B, Vol. 112, No. 49, 2008 15541

coefficient, spectral shift, and lifetime of the hole at the TPA core, arising from the hole stabilization and the increase in rigidity by the peripheral dendrons. 2. Experimental Section Materials. One of the control samples of ordinary triphenylamine (TPA) was purchased from Wako Chemical and used without further purification. The dendrimer cores tris[4-(2thienyl)-phenyl]amine (TPAc-H) and tris[4-(5-hexyl-2-thienyl)phenyl]amine (TPAc-hex) were prepared by previously reported procedures.9c Dendrimers of up to four generations of TPAcDPA G1, TPAc-DPA G2, TPAc-DPA G3, and TPAc-DPA G4 were synthesized by reaction of a TPA precursor with the DPA G1, DPA G2, DPA G3, and DPA G4 dendrons in the presence of titanium(IV) tetrachloride and 1,4-diazabicyclo-[2.2.2]octane. All of the chemical structures are illustrated in Figure 1. Details of the synthesis, including characterization by mass spectroscopy, nuclear magnetic resonance, infrared spectroscopy, and elemental analysis, have been reported in the literature.9c HPLCgrade benzonitrile (99.9%) and carbazole in the highest grade available were purchased from Sigma-Aldrich and Tokyo Kasei, respectively, and used as received. Pulse Radiolysis. A nanosecond pulse radiolysis system at ISIR, Osaka University10 was used for the kinetic and spectroscopic studies of the radical cations. The schematic of the system is illustrated in Supporting Information, Figure S1. A 27 MeV electron beam with 8 ns duration from a linear accelerator (linac) was used as the irradiation source. The probe light source was a Xe flash lamp (having a continuous spectrum from about 300 to more than 1600 nm). Transient photoabsorption spectra were measured using a Hamamastu PMA-11 optical multichannel analyzer. For measurements of kinetics, the white light was spectrally separated using a monochromator (Ristu MC-10N) and detected with a Si or InGaAs PIN photodiode. The signals were collected by a Sony Tektronics transient digitizer (SCD1000). Benzonitrile solutions were loaded into a quartz cell having a 2 cm optical path length and bubbled with oxygen for at least ten minutes. The concentrations of dendrimer and carbazole were 0.28-0.59 and 20 mmol dm-3, respectively. All experiments were carried out at room temperature. 3. Results and Discussion Radiolytic reactions of benzonitrile (PhCN) solutions of a solute (S) have been reported by Kira and Thomas,11 demonstrating a G value (products per 100 eV absorbed energy) for ionic species of as much as 1.4. The primary reactions in O2bubbled PhCN solution in the presence of a high concentration of carbazole (Cz) and a low concentration of S are summarized below.

PhCN ' PhCN · ++e-

(1)

e- + PhCN f PhCN · -

(2)

PhCN · + + Cz f PhCN + Cz · +

(3)

Cz · + + S f Cz + S · +

(4)

PhCN · - + O2 f PhCN + O2 · -

(5)

First, radical cations of PhCN (PhCN•+) and electrons (e-) are generated via ionization. Subsequently, the electrons are

Figure 2. Transient photoabsorption spectra in O2-bubbled benzonitrile solution of 20 mmol dm-3 carbazole in the presence of 0.43 mmol dm-3 TPAc-DPA G1 obtained by nanosecond pulse radiolysis. Time evolution of the absorption spectra after electron-beam irradiation (a) from 0 to 1 µs delay, and (b) from 1 µs to 5 ms delay. The inset in panel b shows kinetic traces at 830 nm (red line, peak of the carbazole radical cation) and at 1440 nm (green line, peak of the TPAc-DPA G1 radical cation). The blue curve represents the decay of carbazole in benzonitrile solution without TPAc-DPA G1.

captured by PhCN, forming radical anions of PhCN that are denoted PhCN•-. Positive charge is then transferred to a large amount of Cz (20 mmol dm-3), which acts as a mediator of positive charge to assess the extinction coefficient of the radical cation of S (S•+) and to increase the radiolytic yield. Reactions 1-3 occur faster than the time resolution of the present apparatus. Hole transfer from Cz•+ to S with the concentration of the latter (less than 1 mmol dm-3) much lower than that of the former proceeds more slowly after the electron pulse. In the presence of oxygen, an electron is scavenged from PhCN•-, giving rise to spectroscopically inert superoxide radical anions (O2•-). The concentration of saturated O2 in PhCN is 8.5 mmol dm-3,12 which is much higher than that of S, and thus the contribution of PhCN•- to the transient photoabsorption is eliminated at long times. It has been asserted that the triplet excited-state of PhCN (denoted PhCN*) contributes to photoabsorption at short wavelengths (1550 >1550 >1550

3.6 × 104 4.3 × 104 5.0 × 104 >4.8 × 104 >4.8 × 104 >4.5 × 104

0.49 0.48 0.81 >0.76 >0.74 >0.69

1018 1100 1902 2369

0.70 0.80 1.41 1.44

g

g

g

g

a Photoabsorption peak in the visible region. Typical error is (2 nm. b Extinction coefficient. Typical error is 10%. c Oscillator strength obtained by fitting half-Gaussian (low energy side), half-Lorentzian (high energy side) functions to the experimental spectra. d Electronic transitions and oscillator strengths calculated by TD-DFT. The n-hexyl substituents of TPAc-hex were replaced by n-ethyl to reduce the calculation time. The geometries of all radical cations were performed using UB3LYP 6-31G(d,p). The base sets of TD-DFT were the same; however, UB3LYP 3-21G(d) was used for G1 and G2 to reduce the calculation time. e Photoabsorption peak in the infrared. Typical error is (10 nm. No peaks of TPA were found in the infrared region. Spectral peaks of TPAc-DPA G2 to G4 were not observed due to the limited sensitivity of an InGaAs detector. f Not observed. g Not calculated due to the insufficient computer resources.

Figure 4. Extinction coefficients of radical cations of the core and dendrimers assessed by pulse radiolysis. The inset shows a magnified view of the absorption peaks in the visible region.

Figure 5. SOMOs for the radical cations of (a) TPA, (b) TPAc-H, and (c) TPAc-ethyl calculated by TD-DFT with UB3LYP 6-31G(d,p). Note that TPAc-ethyl models TPAc-hex.

absorption peaks and oscillator strengths show the same dependence on experimental observations, and the sums of the oscillator strengths assessed by TD-DFT (1.17 for TPAc-hex•+ and 1.30 for TPAc-H•+) are consistent with the experimental results (1.09 and 1.08, respectively). In comparison with the core TPAc-H•+, the first generation of dendrimer radical cation TPAc-DPA G1•+ had approximately 100 and 350 nm red shifts in the visible and infrared absorption peaks, respectively. The oscillator strengths were roughly doubled for both of these electronic transitions. The red shift and increase in oscillator strength can be explained by a further extension of the π conjugation. As seen in Figure 6a, the SOMO is delocalized from the central nitrogen, benzene, thiophene, and benzene to the carbon atom at the root of the diphenyl of the dendron DPA G1. The calculated photoabsorption peak positions (574 and 1902 nm) increasingly disagree with the experimental observations (602 and 1480 nm), which is in contrast to the coincidence observed for the dendrimer core

Figure 6. Geometries of TPAc-DPA G1 and G2 radical cations optimized by DFT with UB3LYP 6-31G(d,p), with the SOMO superimposed.

(TPAc-H•+). The calculated oscillator strength (fcal) in the infrared was three times larger than in the visible, while the experimental oscillator strength in the infrared was 40% smaller than that in the visible. A good coincidence was found as well for the sums of the oscillator strengths between experimental (2.18) and TD-DFT (1.91). The SOMO of TPAc-DPA G2•+, as shown in Figure 6b, is almost identical to that of TPAcDPA G1•+; however, a small portion of molecular orbital lies in the DPA of second generation. The calculated absorption peaks are located at 590 and 2369 nm, which are 16 and 467 nm red-shifted relative to TPAc-DPA G1•+, although the oscillator strengths did not change so much. A continuous red shift of 10 nm was experimentally observed for the photoabsorption of TPAc-DPA G1 ∼ G4•+ in the visible region, while the red shift in the infrared region, which is expected to be larger than in the visible, was not observed due to the limited detector sensitivity. Accompanying the red shift, a decrease in the oscillator strength from 1.37 for TPAc-DPA G1•+ to 1.08 for TPAc-DPA G4•+ was observed for the electronic transition in the visible region. The decrease of the oscillator strength with the generation number is consistent with the decrease in the slope of Mark-Houwink plots ([η] ) KMR) that was found for the neutral dendrimers in the larger generations.3a It should be noted that the correlation of oscillator strength with the viscosity index (R) of Mark-Houwink plot has been demonstrated for radical cations of polysilanes, where the large R gives a large oscillator strength.21 This is due to the

15544 J. Phys. Chem. B, Vol. 112, No. 49, 2008 elongation of conjugation length for a rodlike conjugated polymer backbone having a large R. From the TD-DFT studies of TPAc-DPA G1•+ and G2•+, the red shift of photoabsorption peaks is understood as a result of increase of conjugation length and the lowering of LUMO and SOMO energy levels. The calculated red shift from TPAc-DPA G1•+ to G2•+ for the absorption peaks in the visible region was 16 nm, while the experiments showed only 3 nm red shift. The suppression of the shift is speculated due to the rigid dendrons and resultant disturbance of conformational stabilization of the core of dendrimers. This is also expected to decrease the oscillator strength for the photoabsorption, where the oscillator strengths of G1•+ and G2•+ obtained by TDDFT are almost the same for the absorption in the visible spectral area. Higher generation of DPA might disturb further the complete conformational stabilization and delocalization of the positive charge at the core, which is supported by the abovementioned Mark-Houwink relationship. No significant dependence of the extinction coefficient on the generation number has been observed for the stilbene core with benzyl ether dendrons.7 On the contrary, we demonstrated for the first time a systematic decrease in the oscillator strength together with a red shift of the photoabsorption peak with an increase in the generation number of the dendrimers bearing rigid dendrons. One finds from Figure 6 that there is a large space for outer reactants to directly attack the SOMO of TPAc-DPA G1•+ and G2•+, leading to ineffective shielding of the peripheral dendrons. This is consistent with the fact that the lifetimes of TPAc-DPA G1•+ and G2•+ are almost the same as that of TPAc-H•+ as shown in Figure 3. Long lifetimes are measured for TPAc-DPA G3•+ and TPAc-DPA G4•+; in particular, the lifetime of the latter is nearly 2 orders of magnitude longer than that of TPAcH•+. The dendrons for higher generation numbers occupy the outer space, thereby shielding the positive charge on the TPA core against the reactants and/or stabilizing the hole against the decomposition. 4. Conclusion The optical properties of radical cations of dendrimers bearing a TPA core with rigid DPA dendrons were investigated at up to four generations by a nanosecond pulse radiolysis. Because of the shell effect by the peripheral dendrons, radical cations of the fourth generation dendrimer survived for 2.7 ms, which is nearly 2 orders of magnitude longer than that of core radical cations without dendrons. Large oscillator strengths arise from electronic transitions of multiple electrons for the photoabsorption of the radical cations of the core and dendrimers. TD-DFT studies quantitatively support these experimental observations. With increasing dendrimer generation, a continuous 10 nm red shift in the visible absorption peaks and a decrease in their oscillator strengths were found. These changes were assumed to be linked to the extension of conjugation and the disturbance of conformational stabilization of the core radical cation by the bulky, rigid, insulating DPA dendrons. Acknowledgment. We greatly appreciate the experimental assistance of Dr. K. Okamoto and Mr. S. Suemine of Osaka University. This work was supported in part by a grant-in-aid for scientific research from the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT). Supporting Information Available: The schematic of a nanosecond pulse radiolysis system, control experiments of pulse

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